The invention relates to a fluidized bed adapted for continuous quenching of steel wires to a temperature of 250.degree. C. at the lowest. As known, a fluidized bed comprises a container that is filled to a certain height with granules that form the fluidized bed. The granules are inert to high temperatures of 1500.degree. C. and more. At the bottom of the granule bed, there is an inlet adapted for blowing a carrying gas upwards into the bed, with an input flow that is as equally as possible distributed over the bottom surface of the bed. Between a minimum and maximum blowing speed, the granules come to whirl up and down and the bed swells up so as to behave like a cooling fluid that can be traversed by the wires without any hindrance. Typical grain materials are silica-, amumina-, or zirconiasand, silicon carbide or ferrosilicon, and typical grain dimensions lie in the range between 0.03 and 0.5 millimeter and typical fluidized bed heights for wire applications lie around 0.3-0.6 meter. The blowing speed into the bed for fluidization thereof depends on the chosen grain type, and typical speeds lie in the range between 0.06 and 0.15 m/sec. In this way the cooling medium receives a heat transmission coefficient towards the wires of the order of 200 to 600 W/m.sup.2.degree. K., which already comes near to the coefficient for cooling liquids. With such cooling medium it is then possible to quench steel wires i.e. to cool with a speed of more than 200.degree. C. per second.
In order to be adapted for the treatment of steel wires, the fluidized bed is further provided with the necessary wire guiding and access means to guide the wire in and out the fluidized bed. In general, the fluidized bed will be arranged for simultaneous and continuous treatment of a number of wires (typical quantities are 10 to 50), which pass side by side through the fluidized bed, in the axial direction of the wires. Typical wire thicknesses vary from 1 to 6 millimeter, and typical carbon contents lie in the range from 0.05 to 1%.
Such a fluidized bed has to maintain its quenching temperature. This means that the quantity of heat that enters the bed via the hot wires and that is given off to the cooling fluid, must also be carried off with the same speed from the fluid. In a fluidized bed, this occurs via the carrying gas that is blown in at a comparatively low temperature, that then takes over the heat from the grains, and that then leaves the bed at the top of it at a higher temperature. The temperature of the fluidized bed is kept as a constant value (notwithstanding any disturbances in the traveling speed and entrance temperature of the wires, and other disturbancies) by regulating of the temperature that influences the entrance temperature of the carrying gas, as described in EP 195.473 (publication number). From the same document it is also known to additionally cool the fluidized bed by means of a secondary system of water cooling pipes that are immersed in the fluidized bed, or by means of blowers that blow cooling air above the fluidized bed.
Such a fluidized bed is however limited with respect to its production capacity (i.e. kg of wire treated per second) per square meter of bed surface, so that a large production also needs a comparatively large fluidized bed. The primary cooling by the carrying gas is limited indeed, because the speed of the carrying gas through the bed cannot be forced up above values above 0.15-0.20 m/sec because the grains would then be blown out of the bed. Consequently, the flow input (m.sup.3 /sec) per square meter of surface (is equal to the speed) has a limit, and the maximum possible difference between entrance and exit temperature of the carrying gas has also a limit that is mainly determined by the imposed quenching temperature. Also the secondary cooling must be limited, because the water pipes cause a disturbance in the fluidization, and if there are too many of them, the fluidized bed appears rapidly to block up and to collapse. When air blow cooling is used above the bed, then the heat drain capacity of the air is too small, and when this air is mixed up with atomized water, then it appears that this causes the upper surface of the bed to cake together.
Moreover, when the production capacity per square meter of bed surface is increased, there is a second problem : the regulability of the fluidized bed temperature. Due to the fact that a larger quantity of steel has to be treated in a smaller bed, larger irregularities in heat input and heat drain must be taken up by a smaller volume, so that there are also large temperature variations that must be taken up by a more powerful and more rapidly reacting regulating system.
It is an object of the invention to provide, with simple means, a fluidized bed with increased production capacity, per square meter of bed surface, and that has an efficient temperature regulating system.
According to the invention, three measures are combined with each other markedly increasing the density of the pipe system (indirect convection cooling), using a pipe system with air instead of water, and transferring the temperature control from the primary to the secondary cooling circuit.
It has been found indeed that the origin of the obstruction and the collapse of the fluidized bed when there are too many water cooling pipes, lies in the residual moistness of the carrying gas that causes condensation against the cooling pipes. This causes a cake-formation around the pipes and this gives the pipes a larger apparent diameter which causes a disturbance in the fluidized bed. From this, it appears that it remains possible to strongly increase the density of the cooling pipes, when care is taken that such condensation is avoided. A possible measure is the use of a very dry carrying gas, but this requires a special preparation of the gas, or else, the choice of the carrying gas is limited. Such gas may, for instance, consist of exhaust gases of a furnace, with a large inherent moistness, and it is often undesirable to be limited in the choice of the carrying gas.
It is now a first measure according to the invention, to sensibly increase the density of the pipes, but then not to send cooling water through the pipes, but ambient air that is sucked in via a ventilator, although air has a smaller cooling capacity than water. However, by the fact that it is air, and not water, that runs through the pipes, the external surface of the pipes do no longer come at the temperature of the cooling water (below 100.degree. C., and, consequently, condensation), but at an intermediate temperature between the temperature of the cooling air (about 40.degree. C. t the exit of the sucking ventilator) and that of the fluidized bed (200.degree. C. or more). There is consequently no longer any condensation of residual moistness and it is possible to pass to a pipe system with much larger density, and which can be fed by a very large flow of cheap ambient air, whereby the lower cooling capacity of the air is largely compensated.
The density of the pipe system of consequently at least such, that its external surface where the cooling by convection of the fluidized bed occurs, takes at least 0.40 m.sup.2 per square meter bed surface, and preferably at least 0.80 m.sup.2. And it is intended, when in use, to send a nominal air flow through it Y which causes a cooling capacity (KW/m.sup.2 bed surface) of the convection cooler that amounts to at least twice, and preferably four times, the cooling capacity of the primary cooling by the carrying gas. The secondary cooling system must not necessarily have the form of a number of pipes, but can also take other forms, in so far as the system is based on indirect convection cooling, i.e. cooling through a separating wall with convection on either side thereof.
Further according to the invention, and as a second measure in combination with the measure above, the control of the temperature of the fluidized bed is transferred from the primary cooling circuit, with the carrying gas, to the secondary cooling circuit, with the indirect convection cooling with air. This is now easily feasible by control of the air flow that can be obtained at cold temperature and without any limit from the ambient air. Flow control of a water cooling system is much more difficult because this is continuously disturbed by steam formation. Due to the fact that according to said first measure, the bulk of the cooling has been transferred from the primary to the secondary circuit, the steering with the secondary cooling, from zero to the nominal cooling capacity, provides a very strong regulating system for the temperature.
The cooling capacity of the convection cooler, fed with air that is sucked in by a ventilator, can further be increased by injecting, in the air stream through the convection cooler, either in the cooler itself or in the supply duct, an atomized liquid, preferably water. Then it is possible to regulate the temperature of the bed by varying the flow, either of the cooling air, or of the liquid injection, or both. In fact, by acting on the injection of an atomized liquid, the specific heat C.sub.p of the cooling air is controlled. This specific heat is at its lowest level when the air is completely dry, but by injection of an atomized liquid, the vaporizing heat for the very small drops power unit of volume is added. In general terms, by varying the flow of the cooling air and/or of the liquid injection, a variation is produced of the product of the flow with the specific heat of the air stream. This product H is called hereinafter the "specific heat C.sub.p (in Joule per m.sup.3 and per .degree.C.) multiplied by the flow (in m.sup.3 per sec.). H is consequently a magnitude in Watt per .degree. C.
Accordingly, in more general terms, the convection cooler has an inlet that is connected with an air source, and the specific heat flow H of the air stream through the convention cooler is variable, and the convection cooler comprises a regulator for keeping the fluidized bed temperature at a constant value, by varying said specific heat flow.
Such a regulator will consequently, according to the general principles in control engineering, comprise a feeling device of the temperature of the fluidized bed, that produces a signal that is representative for that temperature, and a comparator, where said temperature is compared with an adjusted desired temperature and where a correction signal is generated that is representative of the observed deviation, to which is possibly added the integral and/or the derivative over the time of such difference (in the well-known) P, PI, PD or PID regulating systems), and a correcting device where said correction signal is transformed into a variation of a magnitude by means of which the temperature is regulated (in this case the flow of air and/or the liquid injection).
Although it is not always necessary to avoid oxidation during quenching, it is often desirable, and sometimes also absolutely necessary to keep the fluidized bed in a non-oxidizing atmosphere. In this case, a conventional non-oxidizing carrying gas is used, and the fluidized bed and the atmosphere above is as much as possible separated from the external atmosphere, for instance by means of a casing around the fluidized bed that is as closed as possible (but having the necessary passages for the carrying gas and the wires). In a cheap and simple way it is then possible to have the carrying gas supplied from a combustion furnace, in which combustion takes place with a small shortage of oxygen, and of which the exhaust gas, before being blown in as a carrying gas, is passed through a cooling device first, in which the gas is cooled down to a temperature not below 120.degree. C. in order to avoid condensation of the water in the exhaust gas. In this case, the system of the invention is extremely well suited, because the temperature variations of this exhaust gas, as a carrying gas, cannot cause much disturbance any more on one hand, the inlet temperature of this gas has no longer to be controlled as a steering factor for the temperature, and on the other hand, there is the strong regulating system in the secondary cooling system that takes up such temperature variations.
The system according to the invention, and in which the fluidized bed is kept in a non-oxidizing atmosphere, and in which the carrying gas comes from a furnace with uncomplete combustion, is extremely adapted for the quenching operation when continuously patenting steel wires. In such process, the wire is firstly continuously passed through an austenitizing furnace, in which the wire is heated up to a temperature ranging between 900.degree. C. and 1050.degree. C., and then, on exit from the austenitizing furnace, is immediately quenched to a temperature ranging from 530.degree. C. to 570.degree. C. Preferably, the exhaust gas of the austenitizing furnace is used. In this case, the maximal heat drain capacity of the carrying gas per m.sup.2 of bed surface is limited to about 25 KW. Owing to the presence of the strong secondary convection cooling, it is not necessary to design the bed for maximal cooling, so that a larger freedom exists for the design, and the bed can be designed for a heat drain of 10 to 15 KW per m.sup.2 bed surface. The nominal flow of the secondary air cooling is then designed to a value that amounts to more than four times the above value, for instance five times, and in any case more than 50 KW/m.sup.2, for instance 75 KW/m.sup.2.